Proximity-based applications and services represent an emerging social-technological trend. In 3GPP, a Proximity Services (ProSe) capability in LTE is introduced in Rel-12. It is also known as device-to-device (D2D) communication. D2D communication would allow the 3GPP industry to serve this developing market, and will at the same time serve the urgent needs of several Public Safety communities that are jointly committed to LTE. ProSe normative specification is important for enabling economy of scale advantages (i.e., that the resulting system can be used for both Public Safety and non-Public-Safety services, where possible).
A D2D user equipment (UE) may be interchangeably referred to as a ProSe capable UE. A D2D discovery capable UE may be interchangeably referred to as a UE capable of ProSe direct discovery. A D2D direct communication UE may be interchangeably referred to as a UE capable of ProSe direct communication. The radio link and carrier frequency that is used for D2D operation (e.g., ProSe direct communication and/or ProSe direct discovery between UEs) is referred to as sidelink. For example, the UL carrier in Frequency Division Duplex (FDD) or UL subframes in Time Division Duplex (TDD) in a cell can be used as sidelink (i.e., for configuring resources such as subframes for ProSe operation). The cellular transmission and reception may also be interchangeably referred to as Wide Area Network (WAN) transmission and reception, respectively.
In 3GPP, the soft buffer sharing issue is discussed in R1-144524, “LS on soft buffer management for D2D communications,” 3GPP. Based on the 3GPP RAN1 discussion, 3GPP has the following agreements. There is no standardized mechanism defined for D2D communication and discovery to share the soft buffer already defined for cellular communications. A UE with limited reception capabilities shall at a given time first prioritize downlink reception (i.e., reception of WAN signals), followed by sidelink communication reception, sidelink discovery reception on carriers configured by the eNodeB, and sidelink discovery reception on carriers not configured by the eNodeB.
It has been proposed to only use discontinuous reception (DRX) occasions in CONNECTED or IDLE to perform both ProSe discovery and communication reception in order to save the total required memory and/or processor resources at the UE. This approach, however, would require that the WAN transmission be multiplexed with the D2D transmission in time division multiplexing (TDM) manner. With the TDM approach, the D2D transmission cannot be concurrently received with the WAN transmission at the UE. Furthermore, the latency of transmitting data on both WAN link and sidelink may also increase. Thus, such a TDM method may not be suitable for latency critical or delay sensitive services.
It has also been proposed to not consider soft combining for D2D communication and discovery in order to save the required memory and/or processor resources at the UE. When soft combining is not considered, however, link level simulation results show that about 2-3.5 dB degradation is observed. Consequently, a lot of resources are wasted. Furthermore, without soft combining, the D2D coverage will shrink due to the performance degradation. In addition, the bad performance of lower layers (e.g., physical layer) may trigger higher layer transmission, and may further worsen the experience of D2D service. Thus, there is a need to improve the spectrum efficiency and reception performance for the D2D capable UE with limited reception capability.